In order to obtain high information density on a compact carrier, a known practice consists in utilizing an optical disk provided with at least one layer whose physical characteristics vary as soon as it is exposed to radiation. The information is recorded on said layer by means of a focused light beam in the form of diffracting microelements which can subsequently be detected by means of another focused light beam for reading the information. In order to enable users to perform easy and frequent handling operations, the disk (which may be either prerecorded or blank and which will be commercially available) must be provided with protecting means in order to prevent any influence of dust particles, finger marks and the like on the disk and also in order to guard against any impairment of the radiation-sensitive layer.
Examples of disks of this type are described in French patents No. 2,393,394 and No. 2,355,337 which recommend the use of adhesives for bonding a cover to an information carrier consisting of an optical disk by means of spacers. However, the adhesives must be judiciously chosen, not only as a function of their compatibility with the materials of the subtrates and of the sensitive layer but also as a function of their strength and ease of utilization. Furthermore, the process of bonding with adhesives is neither well suited to large-scale production nor conducive to a high standard of reliability.
A more suitable method consists in assembling the components by welding with heating and melting of part of the material of the components to be assembled, with or without an additive substance. Disks of thermoplastic material are particularly well suited to this method. The different known methods of welding of thermoplastics can be contemplated such as, for example, methods which make use of heating blades, friction, high-frequency electric fields, laser beams and ultrasonic waves. The ultrasonic welding technique is particularly well suited to optical disks inasmuch as the very localized heating of very short duration limits the risks of damage to the sensitive layer in the vicinity of a weld fillet or raised ring-shaped zone to be welded.
However, many disks obtained by welding have a deformation of convex shape which is liable in some instances to attain several millimeters. One reason for this phenomenon lies in the thermal shrinkage or contraction at the time of cooling of the weld-fillet material after heating of this latter in order to form the weld and subsequent solidification. During cooling of the weld fillet from the solidification temperature to room temperature, shrinkage of the fillet in fact takes place and this latter compresses the disk in much the same manner as a binding hoop. Furthermore, the disk constitutes a thin structure and, if this compression exceeds a critical value, it produces a buckling stress or curvature of the disk and subsequent deformation.